Semiconductor device including semiconductor memory element and method for producing same

ABSTRACT

A wafer, in which a plurality of rectangular regions are defined on the face of the wafer by streets arranged in a lattice pattern, and a semiconductor memory element is disposed in each of the rectangular regions, is divided along the streets to separate the rectangular regions individually, thereby forming a plurality of semiconductor devices. Before the wafer is divided along the streets, a strained layer having a thickness of 0.20 μm or less, especially 0.05 to 0.20 μm, is formed in the back of the wafer. The strained layer is formed by grinding the back of the semiconductor wafer by a grinding member formed by bonding diamond abrasive grains having a grain size of 4 μm or less by a bonding material.

FIELD OF THE INVENTION

This invention relates to a semiconductor device including asemiconductor memory element, such as DRAM (dynamic random accessmemory) or a flash memory, and a method for producing the semiconductordevice.

DESCRIPTION OF THE PRIOR ART

In the production of a semiconductor device, as is well known amongpeople skilled in the art, a plurality of rectangular regions aredefined by streets arranged in a lattice pattern on the face of asemiconductor wafer, and a semiconductor element is disposed in each ofthe plural rectangular regions. Then, the back of the wafer is ground todecrease the thickness of the water. Then, the wafer is cut along thestreets to separate the plural rectangular regions individually, therebyforming semiconductor devices. In a mode called dicing before grinding,the wafer is not completely cut along the streets, but grooves of arequired depth are formed in the wafer along the streets, and then theback of the wafer is ground to bring the thickness of the wafer intoagreement with the depth of the grooves, whereby the rectangular regionsare separated individually to produce semiconductor devices.

In recent times, semiconductor devices have become compact andlightweight. Thus, it is often demanded that the thickness of thesemiconductor device be rendered 100 μm or less and, further, 50 μm orless. When the back of the semiconductor wafer is ground, a strainedlayer is generated in the back of the semiconductor device owing to thegrinding. Such a strained layer decreases the transverse ruptureresistance of the semiconductor device. If the semiconductor device isbrought to such a small thickness, the above strained layer, if leftunchanged, would result in insufficient transverse rupture resistance ofthe semiconductor device. Hence, if the thickness of the semiconductordevice is to be rendered that thin, it is usual practice to applypolishing or etching to the ground back of the semiconductor wafer,thereby removing the strained layer, before separating the semiconductorwafer into individual rectangular regions. By so doing, excessively lowtransverse rupture resistance of the semiconductor device is avoided.

Studies by the inventors have shown that in the case of a semiconductordevice including a memory element, such as DRAM or a flash memory, ifthe strained layer is removed by polishing or etching after the back ofthe semiconductor wafer is ground, the function of the memory element isdeteriorated. The reason is not entirely clear, but the inventorspresume that upon removal of the strained layer, a gettering sink effectascribed to the strained layer is eliminated, with the result thatimpurities, such as heavy metals, are returned to the neighborhood ofthe face of the semiconductor wafer.

SUMMARY OF THE INVENTION

It is a first object of the present invention, therefore, to improve asemiconductor device including a memory element, thereby impartingsufficient transverse rupture resistance without inducing thedeterioration of a memory function even if the thickness of thesemiconductor device is sufficiently small.

It is a second object of the present invention to provide a method forproducing a semiconductor device including a memory element, thesemiconductor device having sufficient transverse rupture resistancewithout inducing the deterioration of a memory function even if thethickness of the semiconductor device is sufficiently small.

The inventors diligently conducted studies and experiments, and havefound that if a strained layer considerably thinner than a conventionalstrained layer is intentionally rendered present in the back of asemiconductor device, sufficient transverse rupture resistance can beimparted without deterioration of the function of a memory element.

That is, according to a first aspect of the present invention, there isprovided, as a semiconductor device including a semiconductor memoryelement which attains the above-mentioned first object, a semiconductordevice including a semiconductor memory element, wherein a strainedlayer having a thickness of 0.20 μm or less has been formed on the backof the semiconductor device.

The thickness of the strained layer is preferably 0.05 μm or more. Thestrained layer preferably has been generated by grinding the back of thesemiconductor device by a grinding member formed by bonding diamondabrasive grains having a grain size of 4 μm or less by a bondingmaterial.

According to a second aspect of the present invention, there isprovided, as a method for producing a semiconductor device which attainsthe above-mentioned second object, a method for producing asemiconductor device, comprising dividing a wafer, in which a pluralityof rectangular regions are defined on the face of the wafer by streetsarranged in a lattice pattern, and a semiconductor memory element isdisposed in each of the rectangular regions, along the streets toseparate the rectangular regions individually, thereby forming aplurality of semiconductor devices, the method further comprisingforming a strained layer having a thickness of 0.20 μm or less on theback of the wafer before dividing the wafer along the streets.

The thickness of the strained layer is preferably 0.05 μm or more. It ispreferred to form the strained layer by grinding the back of thesemiconductor wafer by a grinding member formed by bonding diamondabrasive grains having a grain size of 4 μm or less by a bondingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a typical example of a wafer towhich a preferred embodiment of a manufacturing method according to thepresent invention is applied.

FIG. 2 is a schematic view showing a state in which the back of thewafer is ground in the preferred embodiment of the manufacturing methodof the present invention.

FIG. 3 is a partial sectional view showing a preferred embodiment of asemiconductor device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a wafer to which a preferred embodiment of a manufacturingmethod according to the present invention is applied. A wafer, entirelyindicated at a numeral 2, is formed of silicon, and is circular exceptan orientation flat 4. On the face 6 of the wafer 2, a plurality ofrectangular regions 10 are defined by streets 8 arranged in a latticepattern. A semiconductor element including a memory element, such asDRAM or flash memory, is formed in each of the rectangular regions 10.Since such a wafer 2 per se is well known among people skilled in theart, its detailed explanation is omitted herein.

In the present invention, it is important to grind the back 12 of thewafer 2, thereby forming a strained layer of a required thickness on theback of the wafer 2. FIG. 2, which shows the manner of grinding the back12 of the wafer 2, is referred to for the purpose of explanation. Beforegrinding the back 12 of the wafer 2, a suitable protective sheet 14 ispasted to the face 6 of the wafer 2. The wafer 2, which has the suitableprotective sheet 14 pasted to the face 6 thereof, is rendered facedown,and placed on a chuck means 16. The chuck means 16, which is rotatedabout its central axis extending substantially vertically, has asubstantially horizontal upper surface, and many suction holes (notshown) are formed in this upper surface. The wafer 2 is placed on theupper surface of the chuck means 16, and the suction holes are broughtinto communication with a vacuum source (not shown), whereby the wafer 2is attracted onto the upper surface of the chuck means 16. A grindingmeans 18 is caused to act on the back 12 of the wafer 2. The grindingmeans 18 includes a support disk 22 fixed to a rotating shaft 20extending substantially vertically. To a peripheral edge portion of thelower surface of the support disk 22, a plurality of arcuate grindingmembers 24 are fixed with spacing in the circumferential direction.Instead of the plural arcuate grinding members 24, a single grindingmember extending continuously in a toroidal form can be fixed to thesupport disk 22. In grinding the back 12 of the wafer 2, the chuck means16 is rotated, and the grinding means 18 is rotated at the same time.The grinding member 24 of the grinding means 18 is pressed against theback 12 of the wafer 2, and the grinding means 18 is moved in ahorizontal direction with respect to the chuck means 16. When the back12 of the wafer 2 is ground in this manner to decrease the thickness ofthe wafer 2 to a required value, microcracks are formed in the back 12of the wafer 2 owing to the grinding, and such microcracks constitute astrained layer 26 (FIG. 3). In FIG. 3, the wafer 2 stripped of theprotective sheet 14 from its face 6 is illustrated in an upright state,and the strained layer 26 constituted of the microcracks formed in theback 12 is indicated by cross-hatching.

In the present invention, it is important to form the strained layer 26having a thickness t of 0.20 μm or less, preferably 0.05 to 0.20 μm, onthe back 12 of the wafer 2. To form the strained layer 26 of such amarkedly small thickness, it is recommendable, according to theinventors' experience, to use the grinding member 24 formed by bondingdiamond grains having a grain size of 4 μm or less by a suitable bondingmaterial such as a vitrified bond or a resin bond. The thickness t ofthe strained layer 26 constituted of the microcracks can be measured byobserving the back 12 of the wafer 2 under a transmission electronmicroscope.

In the above-described manner, the back 12 of the wafer 2 is ground toform the strained layer 26 of the required thickness. Then, the wafer 2is separated along the streets 8, whereby a semiconductor devicecomposed of each of the rectangular regions 10 is completed. To separatethe wafer 2 along the streets 8, it suffices to cut the wafer 2 alongthe streets 8 by a dicer well known per se.

If desired, prior to grinding of the back 12 of the wafer 2, it ispossible to cut the wafer 2 along the streets 8 to a predetermined depthfrom the face 6 of the wafer 2, thereby forming grooves in the face 6 ofthe wafer 2 along the streets 8, and then grind the back 12 of the wafer2, thereby decreasing the thickness of the wafer 2 to the depth of thegrooves. By this procedure, the rectangular regions 10 can be separatedindividually.

EXAMPLE

A silicon wafer of a shape as shown in FIG. 1 was rendered ready foruse. The diameter of the wafer was 8 inches, and its thickness was 725μm. On the face of the wafer, 750 rectangular regions were defined. Eachof the rectangular regions measured 5×8 mm, and DRAM was formed in eachof the rectangular regions. The back of such a wafer 2 was ground in amanner as shown in FIG. 2 to decrease the thickness of the wafer 2 to150 μm. A grinding member used was formed from diamond grains having agrain size of 4 μm or less bonded by a vitrified bond. Then, the waferwas cut along the streets to produce a semiconductor device comprisingthe rectangular region separated individually. Observation of the backof the semiconductor device by a transmission electron microscope showedthat a strained layer having a thickness of 0.15 to 0.19 μm andconstituted of microcracks was formed in the back of the semiconductordevice. The transverse rupture resistance of the semiconductor devicewas measured. The results are shown in Table 1. The transverse ruptureresistance of the semiconductor device was measured by a ball transverserupture method comprising placing the semiconductor device on acylindrical jig, and pressing a ball against the center of thesemiconductor device. Furthermore, a manufacturer of DRAM was asked totest the memory function of the semiconductor device. The results of thetest are shown in Table 1.

Comparative Example 1

A semiconductor device was produced in the same manner as in the aboveExample, except that the grinding member was formed from diamond grainshaving a grain size of 4 to 6 μm. The back of the semiconductor devicewas observed by a transmission electron microscope. It was found that astrained layer having a thickness of nearly 0.50 to 1.00 μm andconstituted of microcracks was formed in the back of the semiconductordevice. The transverse rupture resistance of the semiconductor devicewas measured. The results are shown in Table 1. Furthermore, amanufacturer of DRAM was asked to test the memory function of thesemiconductor device. The results of the test are shown in Table 1.

Comparative Example 2

A semiconductor device was produced in the same manner as in ComparativeExample 1, except that after the back of the wafer was ground, the backof the wafer was polished over a thickness of 1.20 μm with the use of apolishing tool sold by DISCO CORP. under the trade name of “DRY POLISH”.When the back of the semiconductor device was observed by a transmissionelectron microscope, it was found that a strained layer substantiallydid not exist in the back of the semiconductor device. The transverserupture resistance of the semiconductor device was measured. The resultsare shown in Table 1. Furthermore, a DRAM manufacturer was asked to testthe memory function of the semiconductor device. The results of the testare shown in Table 1. TABLE 1 Thickness of Transverse Decline instrained rupture memory layer (μm) resistance (MPa) function Example0.15-0.19 910 No Comparative 0.50-1.00 620 No Example 1 Comparative Zero920 Yes Example 2

1. A semiconductor device including a semiconductor memory element,wherein a strained layer having a thickness of 0.20 μm or less has beenformed on a back of the semiconductor device.
 2. The semiconductordevice including a semiconductor memory element according to claim 1,wherein a thickness of the strained layer is 0.05 μm or more.
 3. Thesemiconductor device including a semiconductor memory element accordingto claim 1, wherein the strained layer has been generated by grindingthe back of the semiconductor device by a grinding member formed bybonding diamond abrasive grains having a grain size of 4 μm or less by abonding material.
 4. A method for producing a semiconductor device,comprising dividing a wafer, in which a plurality of rectangular regionsare defined on a face of the wafer by streets arranged in a latticepattern, and a semiconductor memory element is disposed in each of therectangular regions, along the streets to separate the rectangularregions individually, thereby forming a plurality of semiconductordevices, further comprising forming a strained layer having a thicknessof 0.20 μm or less on a back of the wafer before dividing the waferalong the streets.
 5. The method for producing a semiconductor deviceaccording to claim 4, wherein the thickness of the strained layer is0.05 μm or more.
 6. The method for producing a semiconductor deviceaccording to claim 4, further comprising forming the strained layer bygrinding the back of the semiconductor wafer by a grinding member formedby bonding diamond abrasive grains having a grain size of 4 μm or lessby a bonding material.